1. Field of the Invention
The present invention relates to a technique for enhancing the quality of an image printed by a thermal printhead utilizing error diffusion.
2. Description of the Related Art
As is well known, a thermal printer is provided with a thermal printhead which includes an array of heating regions extending in the primary scanning direction. By selectively heating the heating regions, a desired image can be printed on a recording paper thermosensitively or by thermal transfer using an ink ribbon. As compared with an ink jet printer, such a thermal printer may be advantageous in that the printhead is smaller in size and weight while providing easier maintenance.
On the other hand, as a method for pseudo-half tone processing, dithering is increasingly replaced by error diffusion. The dithering process is one of the area gradation methods. In this method, for a given area including a matrix of print dots, the ratio of on-dots (black dots for example) to off-dots (white dots for example) is adjusted to change the shades in the image. In the dithering, however, one print dot corresponds to one pixel of the image. Therefore, a large number of xe2x80x9coff-pixelsxe2x80x9d may be produced, which may degrade the resolution. The error diffusion, the details of which will be described later, can solve the above-described problem to enhance the image quality more effectively than the dithering process.
The error diffusion is conventionally utilized also for a thermal printer. However, unlike the multi-value error diffusion utilized for an ink jet printer, the error diffusion conventionally utilized for a thermal printer is two-value error diffusion. Specifically, in a conventional thermal printer utilizing the error diffusion, one print dot corresponds to one pixel, and only two print output levels (1 and 0 representing black or white for example) are utilized for the error diffusion.
Specifically, the two-value error diffusion may be performed as follows. First, the half tone value of a first pixel under control is compared with a predetermined threshold value to determine whether the pixel should be made black or white. If the first pixel is determined as white, the tone of the first pixel becomes brighter than the actual tone of the print data. Such a difference (error) from the threshold value is reflected in making the black or while choice for a next pixel so that the next pixel is more likely to be determined as black. In this way, an error generated in the determination of one pixel is xe2x80x9cdiffusedxe2x80x9d to a next pixel on the same line or a next line for making the black or white determination for the next pixel. By repeating these process steps one after another, the errors become negligible when the pixels in a certain area are totally viewed.
As described above, a conventional thermal printer utilizes the two-value error diffusion. Generally speaking, however, half tone representation of an image becomes more sophisticated as the number of values utilized for the error diffusion increases. In this regard, the conventional thermal printer has room for improvement for enhancing the print image quality.
It is therefore an object of the present invention to provide a thermal printer capable of performing error diffusion utilizing three or more values for obtaining a high quality print image.
According to a first aspect of the present invention, there is provided a thermal printer comprises a thermal printhead, a transfer assembly, and a controller. The thermal printhead includes a row of heating regions arranged in a primary scanning direction, and a driver for selectively heating the heating regions. The transfer assembly feeds a recording paper in facing relationship to the row of heating regions of the printhead in a secondary scanning direction perpendicular to the primary scanning direction. The controller is combined with the driver for causing each of the heating regions to selectively form, on the recording paper, differently sized print dots which include an off-dot, a maximum-size dot, and at least one intermediate-size dot.
Preferably, the controller combined with the driver causes each of the heating regions to selectively form different intermediate-size print dots in addition to the off-dot and the maximum-size dot.
According to a preferred embodiment, the controller combined with the driver controls printing on a pixel-by-pixel basis. Each pixel comprises a matrix of print dots which includes at least two rows of print dots located adjacent to each other in the secondary scanning direction. Each row of print dots includes two print dots located adjacent to each other in the primary scanning direction.
Preferably, the controller combined with the driver is capable of printing each pixel in different output levels which include a lowest output level wherein all print dots in the matrix are the off-dots, a highest output level wherein all print dots in the matrix are the maximum-size dots, a first intermediate output level wherein the two dots in one row of the matrix are intermediate-size dots which are equally sized, and a second intermediate output level wherein the two dots in one row of the matrix are the maximum-size dots while the two print dots in another row of the matrix are intermediate-size dots which are equally sized.
Preferably, the recording paper is transferred in the secondary scanning direction by a pitch which is generally equal to a center-to-center distance between the two print dots in each row of the matrix.
Preferably, each of the heating regions has a width in the secondary scanning direction which is smaller than a total width of two adjacent heating regions in the primary scanning direction.
Preferably, the controller combined with the driver causes each of the heating regions to selectively form the different intermediate-size print dots and the maximum-size print dot by selectively supplying pulses of different widths to each of the heating regions.
The thermal printer may further comprise a thermal transfer ink ribbon fed between the printhead and the recording paper.
Other features and advantages of the present invention will become clearer from the detailed description given below with reference to the accompanying drawings.